The present invention relates to an ink jet recording sheet and a process for its production. More particularly, it relates to an ink jet recording sheet whereby ink absorptivity is good, the diameter of dots is small, the shape of dots is substantially spherical, the image is clear and sharp, the water resistance is excellent and the coated layer strength is high with no substantial falling of powder, and a process for its production.
The ink jet recording system is a system wherein fine droplets of ink are jetted by various operational principles and deposited on a recording sheet such as a paper sheet to form a record of images or letters. By virtue of its excellent characteristics such as high speed, low noise, easiness to produce multi-color images, applicability to a wide variety of record patterns and unnecessity of development or fixation, it has found a wide range of applications in various recording apparatus for figures including Chinese characters and for color images.
Various efforts have been made with respect to the apparatus and ink to make it possible to use wood free paper or coated paper which is commonly used for printing or writing, as the recording sheet to be used for this ink jet recording system. However, along with the improvement in performance of the ink jet recording apparatus, such as high speed printing, full color printing and high sharpness like a photograph and with a progress in expansion of its applications, a high level of image quality has been demanded. Accordingly, it has now been required that the recording sheet has the following high levels of properties.
(1) It is excellent in sheet feeding at the time of recording by a common recording apparatus.
(2) Cockling or strike through of an image recorded on a front surface can be prevented, and printed images of high quality can be obtained within a wide range of temperature and humidity conditions.
(3) The printed dot density and the image density are high.
(4) The coloring properties and sharpness of the images are good.
(5) The shape of printed dots is good.
(6) The absorptivity of ink is good.
(7) The image storage properties such as the water resistance, light resistance and ozone resistance of printed images, are good.
(8) In the case of a coated type recording sheet, the adhesion of the coated layer is good with no substantial falling of powder.
(9) Yellowing of the recording sheet itself scarcely occurs.
(10) As compared with the dot diameter at a monochromatic portion, the dot diameter at a superposed colored portion is not substantially different, and highly fine printed images can be obtained with no substantial bleeding at the superposed colored portion.
An ink jet recording sheet having all of the above properties, is possible only with a coated type ink jet recording sheet i.e. a recording sheet to be used exclusively for ink jet recording, which contains a white inorganic pigment excellent in absorptivity, such as porous synthetic amorphous silica, as the main component in an ink receiving layer. The dot density is preferably 360 dots/inch, more preferably 720 dots/inch and further preferably higher than this, and the smaller the dot diameter, the better. Accordingly, one having a small particle size is used as the white inorganic pigment such as amorphous silica to be contained in the ink receiving layer. Such a highly fine ink jet recording sheet, the larger the dot diameter, the more the bleeding at a superposed colored portion, whereby the sharpness of the recorded images tends to be low. Further, with such a highly fine ink jet recording sheet, a high grade of the image quality is regarded as most important. If the absorptivity of amorphous silica changes, such a change influences substantially the image quality of the ink jet recording sheet. With a highly fine image having a substantial amount of inking, there has been a problem that if the absorptivity of amorphous silica decreases, bleeding at a superposed colored portion tends to be substantial, whereby the image quality is likely to be substantially low.
A highly fine ink jet recording sheet has had a drawback that the surface strength, the water resistance and the bond strength of the ink receiving layer tend to be low, since the ink receiving layer contains amorphous silica having a small particle size, and a water-soluble binder incorporated to the ink receiving layer as an adhesive is captured in pores of the amorphous silica. If the surface strength decreases, falling of powder tends to be substantial i.e. a powder falls from the ink receiving layer of an ink jet recording sheet, whereby the powder is likely to deposit on a cutter blade in the cutting operation or in the winding up operation, thus leading to a trouble in operation, or a paper powder is likely to accumulate on a feed roll of a printer, thus leading to failure in feeding, or in an application to letters or the like, when a printed sheet is folded and inserted into an envelope, a trouble such as falling of powder from the folded line, is likely to result. Otherwise, if the amount of the adhesive is increased to prevent such falling of powder, there has been a problem that the absorptivity tends to deteriorate, and the sharpness of the printed images tends to be low.
Further, if water resistance of an ink receiving layer deteriorates, there has been a problem that the coated layer is likely to peel by friction when the ink jet recording sheet is overlaid on another in a wet state immediately after printing. As described above, in the production of highly fine ink jet recording sheets, there have been many technical problems such as bleeding of a superposed colored portion of a printed image, falling of powder and deterioration in water resistance.
Various methods have been proposed to overcome such technical problems. For example, JP-A-9-263040 proposes to prevent falling of powder by increasing the coated layer strength by incorporating colloidal silica and sodium silicate in a certain specific ratio to the ink receiving layer. However, by the addition of an alkali such as sodium silicate, there has been a problem that the pore surface of amorphous silica tends to dissolve and precipitate during drying to clog pores, whereby the ink absorptivity tends to be low, and bleeding of printed images is likely to be facilitated. Further, there has been a practical problem that amorphous silica dissolves by the addition of an alkali, whereby the viscosity of the coating liquid abnormally increases so that preparation of the liquid becomes impossible.
As mentioned above, it has been known to incorporate an alkali to amorphous silica in order to increase the surface strength of the ink receiving layer of an ink jet recording sheet, but the prior art has disclosed nothing about stabilization of the pore surface of amorphous silica. Namely, in the prior art, no consideration has been made on such a point that amorphous silica undergoes a change in the pore surface state depending upon the number of days passed after the production or the temperature and humidity conditions of the environment during the storage, whereby the ink absorptivity changes to lead to bleeding of printed images or an increase of the viscosity of the coating liquid during the preparation of the coating liquid.
It is an object of the present invention to provide an ink jet recording sheet whereby highly fine printed images free from bleeding can be obtained, by analyzing the causes for the change in the image quality depending upon the number of days after the production of amorphous silica or the temperature and humidity conditions of the environment during the storage and stabilizing the pore surface thereby to stabilize the ink absorptivity.
Another object of the present invention is to provide an ink jet recording sheet which is free from bleeding of printed images or free from an increase of the viscosity of the coating liquid even when an alkali is added and which has improved water resistance and is free from falling of powder.
The present inventors have conducted various studies with respect to ink jet recording sheets and as a result, have found it possible to provide an ink jet recording sheet having stabilized absorptivity and free from bleeding of printed images by presenting an ink jet recording sheet having an ink receiving layer formed on at least one side of a support, wherein the pore volume in the ink receiving layer is at least 0.80 ml/g, and the pore volume of pores with pore radii of from 5,000 to 50,000 xc3x85 in the ink receiving layer is at least 0.40 ml/g. The ink receiving layer preferably contains amorphous silica as the main component and at least one of alkali metal hydroxides as an alkali. As the amorphous silica, particularly preferred is one having a secondary average particle size of from 2.0 to 8.0 xcexcm, a BET specific surface area of from 250 to 400 m2/g, an oil absorption of from 270 to 310 cc/100 g and a bulk specific gravity of from 100 to 250 g/l. Further, it has been found practically preferred that the amorphous silica is of a mixed system of water/amorphous silica or water/alkali/amorphous silica, and when it is formulated into a dispersed slurry having a concentration of from 16 to 20% with a pH of 6.0 to 11.6, the viscosity of the dispersed slurry is from 40 to 300 cps.
Further, the present inventors have found it possible to provide an ink jet recording sheet having stabilized absorptivity and free from bleeding of printed images by presenting an ink jet recording sheet having an ink receiving layer formed on at least one side of a support, wherein the ink receiving layer contains amorphous silica having a pore volume of from 0.80 to 2.00 ml/g in a solid state as dried after being formulated into a dispersed slurry at pH 11. As the amorphous silica, particularly preferred is one having a secondary average particle size of from 2.0 to 8.0 xcexcm, a BET specific surface area of from 250 to 400 m2/g, an oil absorption of from 270 to 310 cc/100 g and a bulk specific gravity of from 100 to 250 g/l. Further, it is practically preferred that the amorphous silica is of a mixed system of water/amorphous silica or water/alkali/amorphous silica, and when it is formulated into a dispersed slurry having a concentration of from 16 to 20% and a pH of from 6.0 to 11.6, the viscosity of the dispersed slurry is from 40 to 300 cps. The ink receiving layer preferably contains at least one of alkali metal hydroxides.
As a process for producing such an ink jet recording sheet, it is preferred to employ a process which comprises treating amorphous silica having a secondary average particle size of from 2.0 to 8.0 xcexcm, a BET specific surface area of 250 to 400 m2/g, an oil absorption of from 270 to 310 cc/100 g and a bulk specific gravity of from 100 to 250 g/l, by at least one method selected from {circle around (1)} a method of leaving it to stand still naturally for at least 10 days, {circle around (2)} a method of leaving it in an environment at a temperature of from 50 to 70xc2x0 C. for at least 3 days, {circle around (3)} a method of leaving it in an environment with a humidity of from 60 to 80% for at least 3 days, and {circle around (4)} a method of leaving it in an environment at a temperature of from 50 to 70xc2x0 C. and with a humidity of from 60 to 80% for at least 2 days, then formulating it into a dispersed slurry, and coating the slurry on a support to form an ink receiving layer.